...instead of access via mainWindow

instead get the color directly from the referred material

* introduce dedicated MaterialEditorModel
* introduce ui file for material editor dialog
* move actions, toolbars & subwidgets to new editor dialog class
* remove now obsolete external custom editor for material color
* remove now obsolete code, files, classes

"WidgetBox" was due to widget selection in QtDesigner, from which this class was copied once

and
* adapt qrc files
* remove obsolete files

Remove obsolete tooltip database (xml & related class). The tooltip database was only used in one spot. To simplify the code, these tooltips can be defined
directly where they are used.

Some of the tooltips are also present in item classes (and therefore duplicate). This may be worth more refactoring in the future.